In the automotive industry, the use of aluminum alloys for automotive applications is increasing due to their desirable combination of properties such as low density, high strength, corrosion resistance, and formability.
Typically, automotive body sheet products are approximately 0.040" in thickness and include 2000 and 6000 series aluminum alloys. Preferred 2000 series aluminum alloys include, but are not limited to, AA2008, AA2010, and AA2036. Preferred 6000 series aluminum alloys include, but are not limited to, AA6009, AA6010, AA6016, and AA6111. Outer body panel applications require high strength, typically to improve dent resistance, and superior surface appearance, to ensure that no discontinuities are visible after paint has been applied. Inner body panel applications do not require the strength levels or surface appearance necessary for outer body panel applications; however, formability is a critical concern because of the complex nature of most inner body panel designs.
With reference to FIG. 1, a typical prior art processing sequence is depicted for making aluminum sheet. Ingots of the aluminum alloy are cast, homogenized and hot rolled to a desired gauge, such as approximately 0.18041. Typically, the homogenized ingot enters the hot rolling step at a temperature of approximately 850.degree. F. to 1000.degree. F. and exits at temperatures generally greater than 650.degree. F. The hot rolled material is then cold rolled to final gauge, solution heat treated, leveled, and formed for its desired end use. U.S. Pat. No. 4,614,552, entitled ALUMINUM ALLOY SHEET PRODUCT, the contents of which are herein incorporated by reference, describes conventional prior art practice for producing AA6111 alloys.
One drawback associated with the use of aluminum alloys for automotive components is the presence of objectionable and/or deleterious surface defects referred to as ridging, roping, or paint brush lines, which appear on the surface of stamped or formed aluminum sheet components. The ridging lines are present on the surface of the automotive component as a series of closely spaced lines in the rolling direction. Typically, the ridging lines are approximately 0.2 microns high and are spaced less than approximately 1 mm apart. The ridging or roping lines appear in the rolling direction only upon application of sufficient transverse strain, as that occurring in typical stamping or forming operations.
This ridging defect is of sufficient severity to be visible in the automotive component after painting. Consequently, the finished surface appearance of these aluminum alloys is objectionable and not suitable for exterior automotive applications. This ridging defect may also serve as a strain concentration site during forming, thus limiting formability.
In view of the problems with automotive components processed using aluminum sheet made with prior art practices, a need has developed to eliminate the ridging line defect in aluminum alloy sheet products. Elimination of this defect should encourage increased application of aluminum alloys.
In response to the preceding, the present invention provides a method of making aluminum alloy sheet product, especially for automotive use, which minimizes or eliminates the occurrence of ridging lines. The inventive method produces an aluminum alloy sheet product which can be stamped or formed and painted for automotive use without an objectionable surface appearance having defects, such as ridging or roping lines.